The present invention is generally directed to material handling vehicles and, more particularly, to an automatic guided vehicle that is capable of automatically loading and unloading a transport, for example, a tractor trailer, a rail car, a flatbed trailer or a shipping container.
Automatic guided vehicles (AGVs) are used throughout the material handling industry to transport loads. The term AGV is commonly used to refer to robust vehicle designs having any of a number of available automated guidance systems. Automatic guided carts (AGCs) is a term commonly used to refer to a less robust vehicle used for similar but less complicated applications. Throughout this application, including the claims, the term AGV shall mean and include both AGV's and AGC's, as well as any other vehicle that is automatically guided.
Current AGV designs generally include a frame with swivel castors located at the four corners of the frame. Other features may include a drive wheel assembly and rigid castors for directional control of the cart. In one current design, two rigid castors are fixed to the frame and located approximately midway between the swivel castors on each side of the cart frame. The two pair of swivel castor axes and the rigid castor axis are generally parallel to each other. The steerable driving unit is attached to the cart frame, generally by way of a plate that is hinged and spring loaded from the cart frame to ensure that the steerable drive wheel maintains adequate traction with the support surface. In another embodiment, a fixed drive wheel propels the AGV and a steerable castor wheel directs the movement of the AGV.
An AGV includes a guidance system that controls its movement. Known guidance systems in use today include wire guidance, laser guidance, magnetic tape guidance, odometry guidance, inertial guidance and optical guidance, and each have their own associated positives and negatives. For example, inertial guidance is susceptible to tracking errors, where the travel distance and direction measured by the AGV differs from the actual distance and direction of travel. Though they can be minimized, tracking errors may compound over long travel distances and the system must adjust for these errors, for example, by utilizing waypoint reference markers (magnetic paint, Radio Frequency Identification (RFID) tags, etc.) along the designated path.
Laser guidance systems use special markers that the AGV senses and uses to control its travel. This type of system is susceptible to obstruction of markers and, most notably, requires markers to be present in any environment of travel. If the path of the AGV is modified, the markers must be physically moved. Further, an AGV with this type of guidance system can only travel in areas that have these special markers, which, in the context of this invention, requires that any transport to be loaded or unloaded include markers.
One difficulty associated with the automatic loading and unloading of a transport is the varying position of the transport in relation to the fixed loading dock position. Transports are usually positioned manually, for example by a driver in the case of a truck. This manual positioning results in an unknown variability in the position of the transport. As a driver positions the trailer at the loading dock, he or she may be unable to perfectly square the trailer with the dock door. This will leave the trailer at a skewed angle in reference to the dock door. Since the angle is unknown and can vary at each positioning at the dock, an AGV cannot effectively guide and deliver loads in the trailer without having the capability of detecting and compensating for this trailer skew. The prior art has addressed this problem by using skid plates to position the transport in relation to the loading docks, however this is a costly and inefficient process. The trailer may also be positioned offset from the optimal position relative to the dock door. In loading wider loads by AGVs, an offset as little as one inch may cause problems during the loading process.
Another difficulty associated with the automatic loading and unloading of a transport is that the AGV must be able to overcome the difference in height between the transport and the dock. Different types of transports, as well as different styles of the same transport, will vary in height. Furthermore, the height of a particular transport is not static; as trailer is loaded the suspension will compress, resulting in a change in the height of the transport. In order to allow robust operation, the AGV must be able to operate with varying transport height and, therefore, varying height differences between the transport and dock. The prior art has addressed this problem by using hydraulic or other types of jacks to stabilize and level the transport, however this is another costly and inefficient process.
The use of a loading ramp between the dock and the transport is often used to ease the transition between the two. However, a steep incline or decline between dock and transport can cause guidance difficulties. For example, an AGV that uses a laser guidance system may lose the target as it moves up an incline, or down a decline, due to the fact that the laser will be pointing either above or below the target.
The variability in position of the transport may prohibit the automatic loading of the truck, and almost certainly will reduce its efficiency. For example, the most efficient loading process positions the loads as closely to each other as possible, and any variability in the expected position of the transport will tend to increase the separation of the loads.
Despite the use of guidance systems to control travel of an AGV, their use in the process of loading and unloading loads from a transport has yet to be satisfactorily addressed in the art.